The present disclosure relates to approaches to digital predistortion, and more particularly to one or more techniques that alone or in combination are applicable to digital predistortion in quickly varying operating conditions.
Power amplifiers, especially those used to transmit radio frequency communications, generally have nonlinear characteristics. For example, as a power amplifier's output power approaches its maximum rated output, nonlinear distortion of the output occurs. One way of compensating for the nonlinear characteristics of power amplifiers is to ‘predistort’ an input signal (e.g., by adding an ‘inverse distortion’ to the input signal) to negate the nonlinearity of the power amplifier before providing the input signal to the power amplifier. The resulting output of the power amplifier is a linear amplification of the input signal with reduced nonlinear distortion. Digital predistorted devices (comprising, for example, power amplifiers) are relatively inexpensive and power efficient. These properties make digital predistorted power amplifiers attractive for use in telecommunication systems where amplifiers are required to inexpensively, efficiently, and accurately reproduce the signal present at their input. However, the computation effort associated with conventional digital predistortion (DPD) adaptation processes is substantial, which adversely effects the predistortion accuracy and robustness due to the limits the computational efforts may place on the number and speed at which DPD coefficients can be derived.
There are several challenges for DPD adaptation implementations involving bi-directional wireless communication (e.g., between a base station (BS) and a user equipment (UE). For example, a UE typically transmits (uploads data) as needed and with only sufficient output power to save battery power. The adjustment of output power level is achieved by changing, for example, the baseband input level, RF gain, and power supply voltage of a power amplifier. Furthermore, the UE transmitter may be controlled (e.g., at the command of the BS) to make use of varying frequency ranges (e.g., channels) for transmission. Yet further, the mode of operation and parameters, such as power level, power supply voltage level, output load of PA (e.g., VSWR) can rapidly change in UE devices. The combination of such variations in operating conditions can change the characteristics of the transmit chain, and more particularly change the non-linear nature of the transmit chain that is being compensated for by the DPD.
Therefore, there is a need for a DPD approach(es) that can provide high performance DPD (e.g., measured by linearity, error rate, spectral mask violation, etc.) over a substantial range of operating conditions, as well being able to adapt to rapid changes of such operating conditions.